The present invention relates to a fuel injection control apparatus, and more particularly to a fuel injection control apparatus which is employed in a fuel injection type engine.
There has been known a fuel injection control apparatus which has an air flow meter for detecting an amount of suctioned air into an engine, a sensor for detecting an engine speed. The prior fuel injection control apparatus determines the amount of the fuel to be injected according to the amount of the detected intake air and the amount of the detected engine speed. In such a prior fuel injection control apparatus, when a vehicle is in the decelerated condition, the amount of the injected fuel is too small compared with the necessitated amount of fuel, and this happens to cause a lean mixture. In other words, when a throttle valve fully closes and the vehicle is in the decelerated condition, a compensation plate within an air flow meter closes by the angle which is greater than that in the proper condition. As a result, the air flow meter indicates the amount of the suctioned air which is less than the amount of the actually suctioned air. If the amount of the detected intake air is small, the amount of the injected fuel also decreases according to the decrease in the amount of the detected air. Thus, if a lean mixture is supplied into an engine, this causes the vibration of a vehicle body to impair the drivability of the vehicle, because an engine brings a torque change.
To dissolve the above-described disadvantages, there has been proposed a fuel injection control apparatus which sets the smallest amount of an injected fuel and regulates the amount of the injected fuel so that the amount of the injected fuel may not become less than the smallest amount. In general, the smallest amount of the injected fuel varies according to an engine speed. In detail, when the engine speed is relatively low, the smallest amount of the injected fuel necessitates a relatively great value in order not to generate the engine torque fluctuation. Contrary to this, when the engine speed is relatively high, it is required to decrease the noxious content contained in the exhaust gas, instead of dissolving the problem of the engine torque fluctuation. Hence, when the engine speed is relatively high, the smallest amount of the injected fuel is necessitated a relatively small value. There has been such a fuel injection control apparatus as the apparatus varies the smallest amount of the injected fuel, which is set according to the variation of the engine speed.
However, even in the case that the smallest amount of the injected fuel varies according to the engine speed, when a throttle valve temporarily opens and the engine speed temporarily increases, the smallest amount of the injected fuel is set to the smallest amount which is set for the high engine speed as the engine speed temporarily increases. After the engine speed temporarily increases, the engine speed promptly drops. In this condition, the smallest amount of the injected fuel for the high engine speed is set and the relatively small amount of fuel is injected. This causes the engine torque fluctuation to generate the vibration of the vehicle body.